Designing wearable UV index sensor devices
UV radiation is a natural part of the environment and even has a beneficial effect in smaller doses. If we tried to eliminate all UV exposure, we would see an increase in skeletal diseases caused by a deficiency of vitamin D that is synthesised by the body with the help of UV. How much of a health benefit we see from UV depends on personal circumstances, since there are strong interactions between UV exposure and skin pigmentation. The key is to maintain UV exposure at an optimum healthy level but not so high that it becomes dangerous.
When developing UV sensing applications, it is helpful to distinguish between the different types of UV. The 1932 Second International Congress on Light defined three distinct types of UV that exist in the 100 nm to 400 nm wavelength range: UVA, UVB and UVC. Only two of these types – UVA and UVB – are important to consumer applications for ambient UV measurements.
The short-wavelength UVC photons from the sun do not penetrate the atmosphere and, for the most part, can be disregarded for use in personal healthcare and wearable computing products. UVC is primarily of interest for industrial applications, for example, to sterilise and disinfect equipment because of UVC radiation's harmful effects on bacteria and other infectious organisms.
UVA and UVB radiation passes through the Earth's atmosphere, although the shorter-wavelength UVB rays, which lie in the 290 nm to 320 nm range, are absorbed more strongly than longer-wavelength UVA rays, which lie in the 320 nm to 400 nm range. In addition to being more prevalent in the atmosphere, UVA penetrates human skin more readily than the more energetic UVB rays, as shown in figure 1.
Figure 1: Types of UV radiation and effects on human skin.
UVA does have some health benefits as it activates melanin pigment already present in the upper skin cells, creating a tan that appears quickly but also fades quickly. But by penetrating into deeper skin layers, UVA also affects connective tissue and blood vessels. According to the World Health Organisation (WHO), the skin gradually loses its elasticity as result of excessive UVA exposure and starts to wrinkle. Recent studies suggest that UVA may also enhance the development of skin cancers, although the mechanisms of this UVA damage are not well understood.
Scientists have known for a long time that UVB rays are more harmful to health than UVA. UVB exposure has been shown to cause damage to DNA, leading to potentially irreversible genetic damage. Mammalian cells have self-repair mechanisms that deal with low levels of DNA damage caused by phenomena such as UV radiation. However, once the damage reaches a certain point, the repair mechanisms cannot keep up, and under normal circumstances the cell triggers its own death, a process known by biologists as apoptosis. For example, this situation occurs when someone receives a bad sunburn. If a skin cell does not correctly perform apoptosis, the potential arises for it to form the core of a cancerous tumour.
Numerous factors lead to significant changes in UV exposure. Higher altitudes reduce atmospheric absorption of UV rays and therefore lead to higher UV exposure. Time of day and seasonality as well as the presence of clouds and dust affect the amount of solar UV radiation that a person encounters while outside. The level of UV radiation varies by approximately four times around the globe, and the situation is complicated by the way in which ozone – which strongly absorbs UVB – is concentrated in the atmosphere. At higher latitudes, less ozone is often present in the atmosphere, which increases the risk of DNA damage from UVB.
The incidence of melanoma tends to be higher for fair-skinned people living in higher latitudes. For example, skin-cancer mortality is six times higher in Nordic countries than in the Mediterranean countries, according to WHO figures. This situation is partly due to fair-skinned people receiving high UV exposure while on holiday in sunnier latitudes.